Polysaccharides represented by cellulose and derivatives thereof are used for a variety of applications. These microporous materials per se can serve as adsorbents, or when these microporous materials undergo a certain chemical modification on their surface, they can be provided with a function such as adsorption or separation.
One of such examples will now be described. Along with the prevalence of enzyme utilization or the development of biopharmaceuticals, the separation and purification of biopolymers such as proteins have become one of the important technical issues. Chromatography is an important means for solving this issue. Chromatography uses a separating agent in which a certain atomic group (often referred to as a selector) that interacts with an intended material or an impurity to be removed is bound to a solid referred to as a matrix.
An extremely important property of a material for separating a biopolymer is that it does not non-specifically adsorb proteins, and therefore, polysaccharides are valued for use as the matrix. Moreover, the presence of a large number of hydroxyl groups in the molecules of polysaccharides allows the selector to be readily bound via ether linkages or ester linkages using the hydroxyl groups as a scaffold, which is also an important factor that makes polysaccharides valued for use as the matrix.
For the separation and purification of a biopolymer, generally, a method is used in which the matrix is bound with a selector having a certain affinity for an intended molecule, and after the adsorption of the intended molecule, the adsorbed intended molecule is liberated and collected in a certain manner. In order to obtain a large amount of the intended molecule, the matrix is demanded to allow binding of a large amount of the selector, and have a porous structure that allows free entry and exit of the intended molecule, in order to allow the selector and the biopolymer having a high molecular weight to be efficiently interacted with each other. In other words, when the matrix is packed into a column to perform size exclusion chromatography, the matrix needs to exhibit an exclusion limit greater than the combined size of the molecule to be purified and the ligand.
This matrix is typically used by being packed as particles into a tube referred to as a column. In recent years, however, attention has been drawn to a new form referred to as a monolith, which is an integral porous material. This is used for the same application by being contained in a container such as a small tube referred to as a capillary, or a column. The monolith can also be used as a filtration membrane, if it has a relatively small thickness and a large area.
One factor for the ease of use of this matrix is its high physical strength, in addition to the selectivity for the target to be separated. Specifically, when a liquid or gas is passed during chromatography or filtration, a matrix having a low elastic modulus will undergo compressive deformation or breakage, and as a result, the flow of the liquid within the chromatographic column will become uneven, or even clogging will occur, leading to a significant deterioration in the separation efficiency of the column. In view of this, high physical strength is an important property, and in this respect, cellulose is an outstanding material among polysaccharides.
Additionally, cellulose has alcoholic hydroxyl groups on the surface as a general characteristic of polysaccharides, and thus, has advantages in that, for example, it is capable of binding a variety of atomic groups by chemical reactions, or is available in abundance and at relatively low cost in the form of a highly pure raw material.
For the foregoing reasons, porous cellulose particles for the main purpose of separating and purifying biopolymers have been developed. Among methods for producing such porous cellulose particles, many methods involve dissolving cellulose in a certain manner, followed by regeneration, while some methods use organic acid esters as starting materials. Direct dissolution of cellulose per se can be difficult in that a special solvent is required, or the viscosity of the solution is very high. On the other hand, the methods using organic acid esters as starting materials utilize the following advantages, for example: organic acid esters can be dissolved in many solvents; organic acid esters of cellulose are industrially supplied with stable quality, at various binding rates or degrees of polymerization with various organic acids; and the ester linkages can be readily broken down to regenerate cellulose.
Patent Literature 1, for example, describes a method for producing such cellulose particles, which includes dispersing, in an aqueous medium, a solution of a cellulose organic acid ester dissolved in an organic solvent such as a halogenated hydrocarbon to form microdroplets of the ester solution, and adding a hydrolysis accelerator such as an ammonium salt thereto to cause hydrolysis of the ester, thus forming cellulose microparticles.
Patent Literature 2 describes a method for producing porous spherical particles, which includes dissolving, in an organic solvent, a cellulose fatty acid ester and a gelling agent for the cellulose fatty acid ester to form a solution, adding the solution into an aqueous medium under stirring to form droplets, further adding a coagulation accelerator to convert the cellulose fatty acid ester contained in the droplets into gel particles, and removing the gelling agent, coagulation accelerator, and solvent from the resulting particles.
Patent Literature 3 describes a method for preparing a particulate cellulose gel, which includes dissolving cellulose in a mixed solution of paraformaldehyde and dimethylsulfoxide, dispersing the solution in a dispersion medium, and then introducing a silicon compound serving as a coagulating agent into the dispersion to cause gelation and coagulation of droplets of the cellulose dispersion.
Non Patent Literature 1 describes that porous particles are formed by dissolving cellulose acetate in a water-soluble organic solvent (a mixed solvent of acetone and DMSO), and dispersing the solution in water, which causes the solution containing cellulose acetate to coagulate upon contact with water.
Non Patent Literature 2 describes that cellulose particles (beads) are obtained by dissolving cellulose diacetate in DMSO, subsequently adding anhydrous sodium sulfate and stirring the mixture, and introducing the mixture into an acid coagulating bath (hydrochloric acid). Moreover, a means for increasing the porosity of the beads by immersing extracted beads in a large amount of warm water to remove sodium sulfate is described.